1. Field of the Invention
This invention relates to a wheel slip controlling system for controlling a frictional force between a tire of a driving wheel and the road surface not only during braking but also during acceleration of a car, and more particularly to a wheel slip controlling system which controls rotation of a driving wheel in response to the output power of an internal combustion engine.
2. Prior Art
An antiskid brake system is conventionally known which controls rotation of a wheel to provide an optimum braking force when braking the speed of a car, without causing a wheel lock. In this type of system, the rotational speed of a wheel (hereinafter referred to as a wheel speed) is controlled, during braking of a car, below a travelling speed of the a car (hereinafter referred to as a car speed) so as to provide a maximum frictional force between a tire of a wheel and the road surface. In particular, where a car speed is indicated by Vs and a wheel speed by V, and the slip ratio S, determined by the following equation EQU S=[(Vs-V)/Vs].times.100,
approaches 10%, a frictional force M between a tire and the road surface becomes maximum and a side force F which is a resistance to a side slip of a vehicle presents an appropriate value. Therefore, the wheel speed is controlled to a speed calculated by the following equation EQU V=(1-.alpha.)Vs-.beta.
where .alpha. is a constant (for example, 0.03) and .beta. is another constant (for example, 4 [km]), in such a manner that the slip ratio S may be held around 10%.
While various slip controls for braking the speed of a car have been proposed so far wherein safety steps are taken so that if a driver brakes a car very quickly, the car may be stopped at a minimum distance without causing a side slip of the car. Proposals for traction control, which is control of a slip during acceleration of a car, have been limited only to such that wherein an output power of an engine is controlled when an acceleration slip occurs at a wheel, and efforts have not been made enough to attain optimum control of the acceleration of a car.
Moreover, conventional traction controls have a drawback that a rapid change in running condition of an internal cumbustion engine may produce vibrations or cause deterioration of exhaust gas emission very quickly since an output of the engine is controlled by ignition timing and air fuel ratio. They also have another drawback in responsibility of control since a time is required until engine torque is lowered after ignition timing control or air fuel ratio control has been initiated.